This invention relates to an improved process for preparing melamine by the conversion of urea or thermal decomposition products thereof, wherein a reaction mixture formed in the conversion is cooled with water or an aqueous medium to form an aqueous product stream containing dissolved or suspended melamine. Solid melamine product is subsequently separated from the aqueous product stream leaving a residual aqueous stream. Since this residual aqueous stream, generally mother liquor remaining after crystallization, still contains some quantity of product melamine, it is desirable to recycle the stream into the melamine process, rather than simply discharging it to waste. However, this residual aqueous stream also contains reaction by-products such as ammeline and ammelide, and if the concentration of these by-products is permitted to build up in the recycled steam, they will contaminate the melamine product crystallized out. It is, therefore, necessary to purify this residual aqueous liquid at some point in its recycle in order to obtain melamine of an acceptable purity.
Such a process is known from U.S. Pat. No. 3,496,176 wherein the residual aqueous liquid remaining after crystallization and removal of solid melamine product is first stripped of ammonia and cooled, and thereafter by-products such as ammeline and ammelide are caused to precipitate by acidification using carbon dioxide or some other acid. The precipitated ammeline and ammelide are removed by filtration, after which the filtrate can be recycled and used for cooling a further quantity of reaction mixture.
Although this known process is capable of preventing a by-product buildup in the recycle stream, the energy consumed in carrying out this purification is rather high. It is, therefore, an object of the present invention to prevent the buildup of reaction by-products in the recycled residual aqueous stream at a significantly lower energy consumption than the prior art, while maintaining acceptable levels of melamine product purity.